Calculations for Functional Safety : Quantities, Formulas and Methods

Whereas in the past, functional safety hardly played a role in many industries, and in the others was essentially ensured by detailed design rules, driven by (negative) experiences, today the trend is moving away from fixed design rules to quantitative requirements and evidence. This undoubtedly promotes innovation and competition, but it also carries the risk of unsafe systems entering the market. The practice of the author as an assessor for functional safety shows again and again, that even experienced safety engineers find it difficult to perform correct calculations. This is often caused by a lack of understanding of the different variables, but just as often it is also due to a lack of knowledge about the calculation tools and methods used (especially FTA tools), coupled with an unjustifiably high level of trust in them. This introduction is primarily intended for prospective and experienced safety engineers, but also to mathematicians or computer scientists, who are entrusted with the development of calculation tools. Reference is occasionally made to standards, however, knowledge of these standards is not presumed. First, the parameters availability, reliability, failure density, failure rate, distribution functions, and MTTF, along with their mathematical foundations and dependencies, are presented in detail, and their practical significance is explained with examples. Then, it is shown how these parameters can also be calculated for complex systems using fault trees and Markov models.